Azurite
The blue that most medieval and Renaissance skies are actually made of. Azurite did the work while ultramarine got the credit, and it has a habit of turning green over the centuries, which is why so many old paintings have skies the colour of a shallow sea.
Origin
A basic copper carbonate, found wherever copper ore weathers near the surface, and mined across Europe — Hungary, Germany, Spain, France — as well as in China and Japan. Because it was local, it cost a small fraction of imported lapis. The preparation is the opposite of ultramarine's: azurite must be ground coarsely, because the finer you grind it the paler it becomes, until at powder fineness it is almost white. This is why medieval azurite passages are visibly gritty under magnification, and why the pigment demands a binder thick enough to hold large particles in suspension. Painters graded it by particle size, the coarsest and deepest blue being the most expensive.
In art history
Azurite is the great workhorse blue of European painting from the Middle Ages to about 1600, and its use is often invisible because it is underneath something. Standard practice was to lay in a sky or a mantle in azurite and finish with a thin ultramarine glaze: the cheap blue supplies the body, the costly blue supplies the surface. When conservators find azurite under an ultramarine passage, they are looking at a workshop economising in a way the patron was probably not told about. In China and Japan the same mineral, called gunjō, was a principal blue for centuries and was used unmixed and unglazed. It appears in four eras here, all of them early, and vanishes from the record as smalt and then Prussian blue displaced it.
In the studio
The failing is chemical and slow. Azurite is metastable: over centuries, in the presence of moisture, it converts to malachite, the green copper carbonate that is its chemical sibling. The transformation is uneven and often partial, so a sky degrades in patches from blue to blue-green to green. Much of what we read as a strange greenish tonality in fifteenth-century painting is not a stylistic choice but a mineral changing phase. Azurite also darkens badly in oil, which is why it was preferred in tempera and glue-based media, and it is destroyed by acids and by heat.
Afterlife
It was already retreating by the seventeenth century and effectively disappeared after Prussian blue arrived. Natural azurite is still sold for conservation and for East Asian painting, where the traditional coarse-ground grades remain in demand. For most purposes it has no modern replacement because nothing else has quite its granular, slightly uneven, mineral surface — the texture is the point, and a smooth synthetic blue cannot imitate it.
Where it appears
Every era and artist in this atlas whose palette includes this pigment.